Conventionally, as small-sized, large-capacitance capacitors, multilayer ceramic capacitors, tantalum electrolytic capacitors, and aluminum electrolytic capacitors are in practical use. A multilayer ceramic capacitor, which uses as a dielectric body a complex oxide such as barium titanate having a large relative dielectric constant, involves a thick-film process, which causes thickness of a dielectric layer to be 1 μm or more. Electrostatic capacitance is in inverse proportion to thickness of dielectric layer and therefore, it is difficult to achieve downsizing and increasing the capacitance at the same time.
On the other hand, a tantalum electrolytic capacitor and an aluminum electrolytic capacitor use as dielectric body, tantalum oxide or aluminum oxide which is obtained by subjecting metal tantalum or metal aluminum to anode oxidation. Since the thickness of the dielectric layer can be controlled by selecting the anode oxidation voltage, it is possible to obtain a thin dielectric layer having a thickness of 0.1 μm or less. However, both tantalum oxide and aluminum oxide have a small relative dielectric constant as compared with that of a complex oxide such as barium titanate, it is difficult to achieve downsizing and increasing in capacitance.
In order to solve the above problems in conventional techniques, many attempts to form a complex oxide thin film on a substrate have been made.
Patent Documents 1 and 2 disclose technique for forming a thin film of barium titanate by chemically forming a metal titanium substrate in a strong alkali solution containing barium ions. Patent Document 3 (related Patent Document: U.S. Pat. No. 5,328,718) discloses technique for forming a thin film of barium titanate on a substrate by alkoxide method. Patent Document 4 discloses a technique for forming a composite titanate film, in which a metal titanium substrate is treated in an aqueous solution of alkali metal to thereby form a titanate salt on the substrate surface and is further treated in an aqueous solution containing other metal ions such as barium ions to thereby substitute the alkali metal with other metals such as barium. Also, Non-Patent Document 1 discloses a technique for obtaining a thin film of barium titanate by hydrothermal-electrochemical technique.
[Patent Document 1] Japanese Patent Application Laid-Open No. S60-116119
[Patent Document 2] Japanese Patent Application Laid-Open No. S61-30678
[Patent Document 3] Japanese Patent Application Laid-Open No. H05-124817
[Patent Document 4] Japanese Patent Application Laid-Open No. 2003-206135
[Non-Patent Document 1] Japanese Journal of Applied Physics Vol. 28, No. 11, November, 1989, L2007-L2009